Aqueous amine based post-combustion CO2 capture is one of the most commonly employed practices for CO2 removal from coal-fired combustion power plants. Extensive efforts have been carried out to seek or develop new amine solvents with the goal of improving CO2 absorption efficiency with lower energy costs. A variety of aqueous amine solutions have been identified as potential CO2 capture solvents, such as monoethanolamine (MEA), N-methyldiethanolamine (MDEA), piperzine (PZ) and 1,6-hexanediamine (HDA) in water. In an amine-based capture process, two equivalents of amine react with the CO2 in the absorber to form one equivalent of carbamate and/or bicarbonate and one equivalent of protonated amine. This ion pair is then regenerated in the stripping process, to release a pure stream of CO2 for compression and two equivalents of amine that are recycled to the absorber (FIG. 1).
The absorption rates for CO2 capture are dependent on kinetics, where faster solvents all for a decrease in the size of the absorber. However, faster solvents will often bind more strongly with CO2, and the regeneration of the carbamate will require more energy in the stripping tower. Even for MEA, PZ, and HDA, as fast kinetic solvents, the cost of capturing CO2 from flue gas of coal combustion is expected to increase the energy consumption by approximately 30%-40%.
Extensive studies have identified solvent improvement as a key point of the energy saving target. One approach is to blend secondary and/or tertiary amines into the aqueous primary amine to improve the rate of CO2 absorption while maintaining high, capacity and reduced energy costs for CO2 removal. However, the potential for in-situ nitrosation of secondary amines by flue gas contaminates (e.g. NO/NO2) to form nitrosamines raises environmental health and safety concerns. Another approach is using enzymes, such as carbonic anhydrases, to enhance the mass transfer by catalyzing the CO2 hydration reaction in alkali salt or tertiary amine solvents. However, enzymes are not likely to survive in the high stripping temperatures (above 100° C.) required for post-combustion CO2 capture. These limitations in the use of secondary and tertiary amines and enzymes prompted investigation into using particles to improve the CO2 capture efficiency of aqueous primary amines.